There is a continuing need to provide more efficient methods for retreading or recapping worn tire carcasses. Energy consumption and time dictate economy in recapping procedures.
While the recapping of tires is an old art, the methods of recapping tires has changed substantially within the last few years. Changes have resulted beause of the basic redesign of the tire itself from bias chord tires to belted and radial tires.
One of the earliest methods of retreading a tire was to place the tire into a mold in which the new tread was formed as a part of the tire. Another method is to attach a premolded tread to the tire carcass.
Tires to be retreaded in molds are buffed and covered with a layer of uncured rubber, and placed into a standard tire retreading mold. In the mold the rubber is shaped into the desired tread pattern by pressurizing the inside of the tire and heating the entire assembly until the uncured rubber is cured, formed and bonded to the old casing by a combination of heat and pressure. An apparatus for this type of operation is shown in U.S. Pat. No. 3,042,966.
Moldless retreading uses a strip of precured and premolded tread supplied in a long continuous strip from an extrusion and molding process. This precured and preformed strip of tread material is vulcanized before it is applied to the tire casing.
In this method, the worn out tire is buffed and the precured and premolded tread rubber is applied to the buffed casing with a layer of uncured raw rubber compound therebetween. The tire is then wrapped up in an adjustable thin steel band and inflated and the entire assembly is then heated and cured in a pressurized and heated chamber.
This retreading process resulted in a recapped tire which inherently had a seam where the two ends of the precured tread joined. An apparatus of this type is shown in U.S. Pat. No. 2,966,936.
More recently, it has been discovered that a seamless, preformed and cured annular tire tread as disclosed in U.S. Pat. No. 3,815,651 may be formed as in U.S. Pat. No. 3,963,393. This tread is then placed about the prepared tire carcass as disclosed in U.S. Pat. No. 3,976,532 and bonded to the tire carcass.
In the various forms of moldless retreading, a flexible rubber envelope encases all or a substantial portion of the carcass and the precured tread. The envelope's interior is subjected to a reduced pressure and its exterior is subjected to an increased pressure to thus hold the tire tread firmly in place while it is bonded to the tire carcass. Such an apparatus is shown in U.S. Pat. No. 3,779,830 which teaches inflation of the carcass during tread replacement.
The tire together with its supporting medium and its encasement is placed in a heated chamber or autoclave for vulcanization of the preformed tread to the tire carcass. The time needed to complete vulcanization is determined by how fast vulcanizing temperature is transmitted to the material to be vulcanized. The time required is a function of the size of the tire, its encasement, and the means used to inflate and hold the tire.
The conventional time required to vulcanize an inflated tire is approximately four hours for an apparatus similar to that shown in U.S. Pat. No. 3,236,709. The heat is permitted to essentially penetrate the treated area only in an unilateral manner.
The tires are conventionally encased in an envelope during preformed tread vulcanization. The placement of the envelope about the tire in a sealed relationship is cumbersome and time consuming.